The main clause of the sentence provides the main idea or the main action.

The main course is preceded by a salad.

The main street is very broad.

The bronchi are the main passageways into the lungs.

The RA is the main branch of the military.

The main street is a busy thoroughfare.

The paperwork is incidental to the main task.

I am from Maine.

The menagerie was the main attraction of the circus.

Flour is the main ingredient in cakes.